1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to a dual panel type organic electroluminescent device that has a reduced number of masks in its fabricating process, and a fabricating method thereof.
2. Discussion of the Related Art
Among flat panel displays (FPDs), organic electroluminescent (EL) devices have been of particular interest in research and development because they are light-emitting type displays that have a wide viewing angle as well as a desirable contrast ratio, as compared with liquid crystal display (LCD) devices. Since a backlight need not be provided in conjunction with such organic EL devices, their size and weight is small, as compared to other types of display devices. The organic EL devices have other desirable characteristics, such as low power consumption, superior brightness and fast response time. When driving the organic EL devices, only a low direct current (DC) voltage is required. Moreover, a rapid response speed can be obtained. It is understood in this field that because the organic EL devices are entirely formed in a solid phase arrangement, unlike LCD devices, they are sufficiently strong to withstand external impacts and also have a greater operational temperature range. Moreover, because fabricating an organic EL device is a relatively simple process with few processing steps, it is much cheaper to produce an organic EL device when compared with LCD devices or plasma display panels (PDPs). In particular, only deposition and encapsulation apparatuses are necessary for manufacturing the organic EL devices.
In an active matrix organic EL device, a voltage applied to the pixel and a charge for maintaining the voltage is stored in a storage capacitor. This allows for a constant voltage driving the device until a voltage of next frame is applied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix organic EL device of low power consumption, high resolution and large area may be made.
FIG. 1 is an equivalent circuit diagram showing a basic pixel structure of an active matrix organic electroluminescent device according to the related art. In FIG. 1, a scanning line 2 is arranged in a first direction, and a signal line 4 and a power line 6 are arranged in a second direction perpendicular to the first direction, thereby defining a pixel region “P.” The signal line 4 and the power line 6 are spaced apart from each other. A switching thin film transistor (TFT) “Ts,” an addressing element, is connected to the scanning line 2 and the signal line 4, and a storage capacitor “CST” is connected to the switching TFT “TS” and the power line 6. A driving TFT “TD,” a current source element, is connected to the storage capacitor “CST” and the power line 6, and an organic electroluminescent (EL) diode “DEL” is connected to the driving TFT “TD.” The organic EL diode “DEL” has an organic EL layer (not shown) between an anode and a cathode. The switching TFT “TS” adjusts a voltage applied to the driving TFT “TD” and the storage capacitor “CST” stores a charge to maintain the voltage applied to the driving TFT “TD”.
When a scan signal of the scanning line 2 is applied to a switching gate electrode of the switching TFT “TS,” the switching TFT “TS” is turned ON, and an image signal of the signal line 4 is applied to a driving gate electrode of the driving TFT “TD” and the storage capacitor “CST” through the switching element “TS” As a result, the driving TFT “TD” is turned ON. When the driving TFT “TD” is turned ON, a current of the power line 6 is applied to the organic EL diode “DEL” through the driving TFT “TD.” As a result, light is emitted. The current density of the driving element “TD” is modulated by the image signal applied to the driving gate electrode. As a result, the organic electroluminescent diode “DEL” can display images having multiple levels of gray scale. Moreover, since the voltage of the image signal stored in the storage capacitor “CST” is applied to the driving gate electrode, the current density flowing into the organic electroluminescent diode “DEL” can be maintained at a uniform level until the next image signal is applied even when the switching element “TS” is turned OFF.
FIG. 2 is a schematic plane view of an organic electroluminescent device according to the related art.
In FIG. 2, a gate line 37 crosses a data line 51 and a power line 42, which are spaced apart from each other. A pixel region “P” is defined between the gate line 37, the data line 51 and the power line 42. A switching thin film transistor (TFT) “TS” is disposed adjacent to the crossing of the gate line 37 and the data line 51. A driving TFT “TD” is connected to the switching TFT “TS” and the power line 42. A storage capacitor “CST” uses a portion of the power line 42 as a first capacitor electrode and an active pattern 34 extending from a switching active layer 31 of the switching TFT “Ts” as a second capacitor electrode. A first electrode 58 is connected to the driving TFT “TD,” and an organic electroluminescent (EL) layer (not shown) and a second electrode (not shown) are sequentially formed on the first electrode 58. The first and second electrodes and the organic EL layer interposed therebetween constitute an organic EL diode “DEL.”
FIG. 3 is a schematic cross-sectional view taken along a line “III—III” of FIG. 2. In FIG. 3, a driving thin film transistor (TFT) “TD” including an active layer 32, a gate electrode 38 and source and drain electrodes 50 and 52 is formed on a substrate 1. The source electrode 50 is connected to a power line 42 and the drain electrode 52 is connected to a first electrode 58. An active pattern 34 made of the same material as the active layer 32 is formed under the power line 42 with an insulating layer 40 interposed therebetween. The active pattern 34 and the power line 42 constitute a storage capacitor “CST.” An organic electroluminescent (EL) layer 64 and a second electrode 66 are sequentially formed on the first electrode 58 and constitute an organic EL diode “DEL.”
For insulating layers, a first insulating layer 30, for example, a buffer layer, is formed between the substrate 1 and the active layer 32. A second insulating layer 36 is formed between the active layer 32 and the gate electrode 38. A third insulating layer 40 is formed between the active pattern 34 and the power line 42. A fourth insulating layer 44 is formed between the power line 42 and the source electrode 50. A fifth insulating layer 54 is formed between the drain electrode 52 and the first electrode 58. A sixth insulating layer 60 is formed between the first electrode 58 and the organic EL layer 64. The third to sixth insulating layers 40, 44, 54 and 60 include contact holes which allow for connections to be made.
FIGS. 4A to 4I are schematic cross-sectional views showing a fabricating process of an organic electroluminescent device according to the related art. In FIG. 4A, a first insulating layer 30, for example a buffer layer, is formed on a substrate 1 by depositing a first insulating material. After forming a polycrystalline silicon layer (not shown) on the first insulating layer 30, an active layer 32 and an active pattern 34 are formed using a first mask process. In FIG. 4B, after sequentially depositing a second insulating material and a first metallic material on an entire surface of the substrate 1, a second insulating layer 36, such as a gate insulating layer, and a gate electrode 38 are formed using a second mask process. In FIG. 4C, a third insulating layer 40 is formed on the gate electrode 38 by depositing a third insulating material. After depositing a second metallic material on the third insulating layer 40, a power line 42 is formed over the active pattern 34 using a third mask process.
In FIG. 4D, after depositing a fourth insulating material on the power line 42, a fourth insulating layer 44 having first to third contact holes 46a, 46b and 48 is formed using a fourth mask process. The active layer 32 can be divided into a channel region 32a, and source and drain regions 32b and 32c by a subsequent doping process. The first and second contact holes 46a and 46b expose the source and drain regions 32b and 32c, respectively. The third contact hole 48 exposes the power line 42. The source and drain regions 32b and 32c are doped with impurities.
In FIG. 4E, after depositing a third metallic material on the fourth insulating layer 44, source and drain electrodes 50 and 52 are formed using a fifth mask process. The source electrode 50 is connected to the power line 42 through the third contact hole 48 (of FIG. 4D) and to the source region 32b through the first contact hole 46a (of FIG. 4D). The drain electrode 52 is connected to the drain region 32c through the second contact hole 46b (of FIG. 4D). The active layer 32, the gate electrode 38 and source and drain electrodes 50 and 52 constitute a driving thin film transistor (TFT) “TD.” The power line 42 and the active pattern 34 are connected to the source electrode 50 and an active layer of a switching TFT (not shown), respectively. In addition, the power line 42 and the active pattern 34 having the third insulating layer 40 interposed therebetween constitute a storage capacitor “CST”.
In FIG. 4F, after depositing a fifth insulating material on the source and drain electrodes 50 and 52, a fifth insulating layer 54 having a fourth contact hole 56 is formed using a sixth mask process. The fourth contact hole 56 exposes the drain electrode 52.
In FIG. 4G, after depositing a fourth metallic material on the fifth insulating layer 54, a first electrode 58 is formed using a seventh mask process. The first electrode 58 is connected to the drain electrode 52 through the fourth contact hole 56 (of FIG. 4F).
In FIG. 4H, after depositing a sixth insulating material on the first electrode 58, a sixth insulating layer 60 having an open portion 62 is formed using an eighth mask process. The open portion 62 exposes the first electrode 58. The sixth insulating layer 60 protects the driving TFT “TD” from moisture and contamination.
In FIG. 4I, an organic electroluminescent (EL) layer 64 and a second electrode 66 of a fifth metallic material are sequentially formed on the sixth insulating layer 60. The organic EL layer 64 contacts the first electrode 58 through the open portion 62 (of FIG. 4H). The second electrode 66 is formed on an entire surface of the substrate 1. The first electrode 58 is designed as an anode. For example, the fifth metallic material can be selected to have high reflectance and low work function because the second electrode 66 should reflect light emitted from the organic EL layer 64 and provide electrons to the organic EL layer 64.
FIG. 5 is a schematic cross-sectional view of an organic electroluminescent device according to the related art. In FIG. 5, first and second substrates 70 and 90, which have inner surfaces facing each other and are spaced apart from each other, have a plurality of pixel regions “P.” An array layer 80 including a driving thin film transistor (TFT) “TD” in each pixel region “P” is formed on an inner surface of the first substrate 70. A first electrode 72 connected to the driving TFT “TD” is formed on the array layer 80 in each pixel region “P.” Red, green and blue organic electroluminescent (EL) layers 74 are alternately formed on the first electrode 72. A second electrode 76 is formed on the organic EL layer 74. The first and second electrodes 72 and 76, and the organic EL layer 74 interposed therebetween constitute an organic EL diode “DEL.” The organic EL device is a bottom type where light is emitted from the organic EL layer 74 through the first electrode 72 and out of the first substrate 70.
The second substrate 90 is used as an encapsulation substrate. The second substrate 90 has a concave portion 92 at its inner center and the concave portion 92 is filled with a moisture absorbent desiccant 94 that removes moisture and oxygen to protect the organic EL diode “DEL.” The inner surface of the second substrate 90 is spaced apart from the second electrode 76. The first and second substrates 70 and 90 are attached with a sealant 85 at a peripheral portion of the first and second substrates.
In an organic ELD according to the related art, a TFT array part and an organic electroluminescent (EL) diode are formed over a first substrate, and an additional second substrate is attached with the first substrate for encapsulation. However, when the TFT array part and the organic EL diode are formed on one substrate in this way, production yield of the organic ELD is determined by a multiplication of the TFT's yield together with the organic EL diode's yield. Since the organic EL diode's yield is relatively low, the production yield of the overall ELD becomes limited by the organic EL diode's yield. For example, even when a TFT is well fabricated, an organic ELD using a thin film of about 1000 Å thickness can be judged as bad due to the defects of an organic emission layer. This results in loss of materials and increased production costs.
In general, organic ELDs are classified into bottom emission types and top emission types according to an emission direction of light used for displaying images via the organic ELDs. Bottom emission type organic ELDs have the advantages of high encapsulation stability and high process flexibility. However, the bottom emission type organic ELDs are ineffective for high resolution devices because they have poor aperture ratios. In contrast, top emission organic ELDs have a higher expected life span because they are more easily designed and they have a high aperture ratio. However, in top emission type organic ELDs, the cathode is generally formed on an organic emission layer. As a result, transmittance and optical efficiency of a top emission type organic ELDs are reduced because of a limited number of materials that may be selected. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film-type passivation layer may fail to prevent infiltration of exterior air into the device.